Ophthalmic lenses, preferably contact lenses and in particular silicone hydrogel contact lenses, which it is intended to produce economically in large numbers, are preferably produced by the so-called mold or full-mold process using rigid molds. In this process, the lenses are produced in their final shape between two mold halves, so that neither subsequent machining of the surfaces of the lenses nor machining of the edge is necessary. Such molding processes are described in more detail, for example in WO-A-87/04390, EP-A-0367513 or in U.S. Pat. No. 5,894,002.
In the known molding processes, the geometry of the contact lens to be produced is defined by a mold cavity between two lens forming surfaces of two mold halves. The edge of the contact lens is likewise formed within the mold, whereby the geometry of the edge preferably is defined by a spatial limitation of UV light used for the polymerization and/or the cross-linking of a lens forming material.
In order to produce contact lenses, in particular silicone hydrogel contact lenses, usually a specific amount of a flowable lens forming material is introduced into the female mold half in a first step. The mold is then closed by putting the male mold half into place. The subsequent polymerization and/or cross-linking of the lens forming material is carried out by means of irradiation with UV light and/or by heating. In the process, either both, the lens forming material in the mold cavity and the excess material in the overflow (or so-called reservoir) are hardened, or only the lens forming material in the mold cavity is hardened, whereas the excess material in the overflow remains as “flash”. In order to obtain fault-free separation of the lens from the excess material, good sealing or expulsion of the excess material must be achieved in the zone in which the two mold halves make contact with each other or in that zone, which defines the spatial limitation of the UV light used for the polymerization and/or the cross-linking of the lens forming material.
After the lens has been formed, the mold is disassembled and the lens removed. Additional processing steps, such as inspection, extraction, hydration, surface treatment and sterilization may finally be performed on the lens before packaging.
Upon polymerization and/or cross-linking of the lens forming material, shrinkage may occur, which may lead to stress in the final lens and/or may lead to delamination of the lens forming material from the mold surface during the molding process, which may result in an uneven lens surface. The problem of shrinkage upon curing (i.e. upon polymerization and/or cross-linking) of lens forming material is known in the art and has been addressed with different technical solutions, such as the following:
U.S. Pat. No. 4,197,266 discloses a method and apparatus for forming ophthalmic lenses, the apparatus comprising cooperating first and second mold surfaces to form a mold cavity. The assembled molds further comprise an annular reservoir connected with the mold cavity. In operation said reservoir provides lens forming material to the mold cavity of the lens, to compensate for shrinkage of the lens material upon curing. In particular the reservoir holds a volume of lens forming material, which during the polymer forming reaction, is kept in a flowable state longer than the lens forming material in the mold cavity.
U.S. Pat. No. 4,565,348 discloses a plastic mold assembly comprising a male and female mold section for casting soft contact lenses, wherein compensation for polymerization shrinkage is attained by providing one flexible and one rigid mold section.
U.S. Pat. No. 4,955,580 discloses a polyolefin mold for casting contact lenses from a polymerizable monomer composition which shrinks on polymerization, wherein said mold comprises male and female mold halves at least one of which has a flexible diaphragm portion which is shaped to provide a surface corresponding to a lenticular surface, said mold halves, when closed, cooperate to define a mold cavity for receiving a volume of said monomer composition, whereby in use said diaphragm is deflected into a cavity under forces exerted on said mold by said monomer composition when polymerized in said closed cavity, thereby compensating for the shrinkage occurring on polymerization.
U.S. Pat. No. 5,466,147 further provides a method and apparatus for molding lenses, such as contact lenses, which have a finished edged, wherein the one mold section comprises a deformable peripheral rim and the other mold section comprises a reversely angled deformable mating surface, so as to accommodate shrinkage during curing.
In spite of all prior art attempts shrinkage is still a problem to be addressed in the manufacture of contact lenses, in particular for silicone hydrogel contact lenses, in particular in the fully automated manufacture of contact lenses using rigid molds, and even more so in the manufacture of silicone hydrogel contact lenses using rigid molds. The present invention therefore is directed to an improved composition for forming a contact lens, in particular for forming a silicone hydrogel contact lens.
Currently almost all silicone hydrogel materials for use as contact lenses are made from formulations which contain a rather large amount of polymerizable monomers (and/or macromers). Due to the reduction in volume occupied by the polymer in comparison to the sum of monomers, the reaction mixture is shrinking in volume upon polymerization. This volume shrinkage is usually in the order of a few %, but may go up to 15% for formulations with a high content of monomers. In conventional contact lens manufacturing technology flexible mold parts are used, which are usually made from polypropylene or similar thermoplastic resins and thus are sufficiently flexible to compensate for the polymerization volume shrinkage. Another contact lens manufacturing technology (e.g. the Lightstream™ technology) uses rigid mold halves. Keeping the distance between the rigid mold parts constant during the polymerization process however may lead to partial or complete delamination of the polymerizing material from the mold surface, potentially leading to surface irregularities which may negatively affect the optical properties of the resulting contact lens.
It is thus an object of the present invention to provide an improved composition and process for forming a contact lens, in particular for forming a silicone hydrogel contact lens, which reduces or avoids surface irregularities and/or uneven lens surfaces, which occur due to shrinkage of the lens forming material upon polymerization and/or cross-linking of the lens forming material in a rigid mold. It is yet a further object of the present invention to provide a contact lens, in particular a silicone hydrogel contact lens, by making use of said composition and process.